1. Field of the Invention
The present invention relates to a disk data reproducing method and a disk data reproducing apparatus suited for reproducing data such as images and sounds recorded in an optical disk, a magneto-optic disk or the like and more particularly to a data reproducing method and a data reproducing apparatus which enable to run special reproductions such as a reverse reproduction quickly.
2. Description of the Related Art
The MPEG (Motion Picture coding Experts Group) method has been proposed as a method for compressing and coding digital image signals recorded in a conventional digital video disk (hereinafter referred to as a DVD). Then, an example of a MPEG encoder will be explained below with reference to FIG. 9.
The MPEG encoder is a type of encoder which is adapted to compress signals by predictive coding, wherein digitized image input signals are formed into blocks each block (MB) being a minimum unit of motion compensative prediction, and motion vectors for the motion compensative prediction are detected per each block in a motion detecting circuit 101.
While this block is predictive-coded by the ensuing predictive coding section, it is classified into four blocks of (1) an intra-block in which DCT (Discrete Cosine Transform) is directly implemented on the image input signals, (2) a forward block in which prediction is made only from the front direction, (3) a backward block in which prediction is made only from the rear direction, and (4) a bi-directive block in which prediction is made from both directions.
That is, a DCT section 103 implements the DCT which is one type of Fourier transform, and a quantization circuit 104 quantizes DCT coefficients of that result. After the quantization, a variable length coding means 109 implements variable length coding by assigning codes whose lengths vary corresponding to a probability of occurrence. An inverse quantization circuit 105 inverse-quantizes the quantized signals, and an inverse DCT section 106 implements an inverse DCT. Then, an output from a frame memory predictor 108 is added to it to reproduce the original image signals. The reproduced image signals are supplied to a subtracter 102 as prediction signals.
Predictive coding signals output from the variable length coding means 109 are multiplexed with prediction mode information and motion vector information in multiplexing means 110. Such multiplexed data is generated at irregular rates, so that it is output to and temporarily stored in a buffer 111 so that its encoding rate becomes constant. Note that it is also possible to control a coding amount by changing a quantization scale factor q of the quantizing means 104 in response to a coding amount stored in the buffer 111 to level off the average of the coding rates.
FIG. 10a shows a structure of inter-frame prediction thus compressed and coded by the MPEG method. In the figure, one GOP (Group Of Pictures) is composed of nine frames for example; one frame of I picture: two frames of P picture and six frames of B picture. Note that GOP is a unit of coding into which one sequence of motion pictures is divided. I picture is a predictive-coded image within the frame, P picture is an inter-frame predictive-coded image predicted with reference to the preceding frame already coded (I picture or P picture) time-wise and B picture is an inter-frame predictive-coded image predicted with reference to two frames of the preceding and succeeding frames time-wise.
That is, as shown in the figure by arrows, I picture I.sub.0 is predictive-coded only within that frame, P picture P.sub.0 is inter-frame predictive-coded with reference to I picture I.sub.0, and P picture P.sub.1 is inter-frame predictive-coded with reference to P picture P.sub.0. Further, B pictures B.sub.0 and B.sub.1 are inter-frame predictive-coded with reference to two pictures of I picture I.sub.0 and P picture P.sub.0, and B pictures B.sub.2 and B.sub.3 are inter-frame predictive-coded with reference to two pictures of P picture P.sub.0 and P picture P.sub.1. Pictures thereafter are created through the predictive-coding in the same manner.
By the way, in decoding the pictures thus predictive-coded, although I picture may be decoded by itself because I picture is predictive-coded within the frame, P picture needs the preceding I picture or P picture in decoding it because P picture is predictive-coded with reference to the preceding I picture or P picture, and B picture needs the preceding and succeeding I picture or P picture in decoding it because B picture is predictive-coded with reference to the preceding or succeeding I picture or P picture. Then, the pictures are rearranged as shown in FIG. 10b so as to be able to first decode those pictures needed in subsequent decoding.
As shown in the figure, this rearrangement is made so that I picture I.sub.0 precedes B pictures B.sub.-1 and B.sub.-2 because I picture I.sub.0 is necessary in decoding B pictures B.sub.-1 and B.sub.-2, so that P picture P.sub.0 precedes B pictures B.sub.0 and B.sub.1 because P picture P.sub.0 is necessary in decoding B pictures B.sub.0 and B.sub.1, so that P picture P.sub.1 precedes B pictures B.sub.2 and B.sub.3 because P picture P.sub.1 is necessary in decoding B pictures B.sub.2 and B.sub.3, and so that I picture I.sub.1 precedes B pictures B.sub.4 and B.sub.5 because I picture I.sub.1 is necessary in decoding B pictures B.sub.4 and B.sub.5.
While I pictures, P pictures and B pictures are recorded in the DVD in the sequence as shown in FIG. 10b, their coding amount is not constant among each picture and varies corresponding to a complexity and flatness of the image since those pictures are predictive-coded as described before. Then, in order to be able to readily handle the data, the data are recorded by means of a sector which is defined by a certain coding amount in recording those pictures once in the DVD. FIG. 11 shows a mode for recording the data by means of the sector, wherein I picture I.sub.0, for example, is recorded in Sector m, Sector (m+1) and a partial area of Sector (m+2), and B picture B.sub.-2 is recorded in the remaining area of Sector (m+2) and Sector (M+3). Thereafter, each picture is recorded sequentially in respective sectors, and one GOP is recorded in Sectors m through (m+13) in this example. However, GOP is not always recorded in such number of sectors, and generally the number of sectors in which one GOP is recorded varies because the coding amount varies due to the complexity and flatness of each image.
The data in sector units read out of the DVD is stored temporarily in storage means which is represented imaginarily as having a ring shape and is called a ring buffer. Operations of a read pointer and a write pointer in the ring buffer will be explained below with reference to FIG. 12. In FIG. 12a, the read pointer RP is positioned at an address position a1 in the ring buffer and the write pointer WP is positioned at an address position b1 slightly before a1. The data in the sector is supplied to the decoder as the read pointer moves clockwise in the figure, reading the data out of the ring buffer.
The write pointer WP is controlled so as to be positioned slightly before a1 time-wise to increase an unread area (URD) as much as possible and not to be short of data to be reproduced. Accordingly, an already-read area (ARD) turns out to be a small area between a1 and b1, and it is also possible to control the pointers so that this area becomes zero. FIG. 12b shows a state wherein the read pointer RP has advanced to read the data out of the ring buffer, advancing its address position from a1 to a2. Due to that, the URD area has become smaller correspondingly, increasing the ARD area on the other hand.
Then, data is written into the ring buffer while advancing the write pointer WP clockwise so that the unread area URD increases as shown in FIG. 12c. The data here is new data read out of the disk. Thereby, the address position of the write pointer WP advances from b1 to b2, reducing the ARD area and increasing the URD area that much. Thus, a large URD area is always maintained in the ring buffer by making such control.
However, there has been a problem that when special reproductions such as a reverse reproduction is to be made when data in sector units recorded in the DVD is being read out of the ring buffer to reproduce video signals, the normal reproduction cannot be switched to the reverse reproductions smoothly. This is so because there exists almost no data to be reproduced in the reverse direction (i.e. data in the ARD area) in the ring buffer at the moment when the mode is switched to the backward reproduction, and it is necessary to wait for a supply of data read by accessing the DVD. That is, although it is necessary to read data of the preceding GOP time-wise following the current GOP by accessing the DVD by the pickup in order to decode video signals to be displayed on the display section during the reverse reproduction, it takes time to read the data because that reading is done mechanically, and it also takes time to decode pictures composing the read GOPs to obtain video signals.
The reason why it takes time for decoding pictures will be explained below. Assume here that a GOP preceding the current GOP is composed of I picture I.sub.0 through B.sub.5 as shown in FIG. 10a. Then, in order to perform the reverse reproduction, it is necessary to display an image of decoded B picture B.sub.5 after an image of decoded I picture I.sub.1 of the current GOP and to display, following that, images of decoded B picture B.sub.4, P picture P.sub.1, B picture B.sub.3, B picture B.sub.2, P picture P.sub.0, B picture B.sub.1, B picture B.sub.0 and I picture I.sub.0.
Because B picture B.sub.5 and B picture B.sub.4 are predicted with reference to I picture I.sub.1 and P picture P.sub.1, data of I picture I.sub.1 and P picture P.sub.1 are necessary for decoding them. However, because P picture P.sub.1 is predicted with reference to P picture P.sub.0, and P picture P.sub.0 is predicted with reference to I picture I.sub.0, it is necessary to decode P picture P.sub.0 with reference to I picture I.sub.0 and to decode P picture P.sub.1 with reference to P picture P.sub.0 after all. It is thus necessary to make reference to I picture I.sub.1 and P picture P.sub.1 to decode B picture B.sub.5 and B picture B.sub.4, so that it takes time for decoding the pictures.
Accordingly, it is an object of the present invention to provide a data reproducing method and a data reproducing apparatus which enable to run a special reproduction such as the reverse reproduction quickly.